Fix mingw32 build


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35177 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/System/Win32/Program.inc b/lib/System/Win32/Program.inc
index 7cc14a2..60e9e41 100644
--- a/lib/System/Win32/Program.inc
+++ b/lib/System/Win32/Program.inc
@@ -100,6 +100,7 @@
 
 #ifdef __MINGW32__
   // Due to unknown reason, mingw32's w32api doesn't have this declaration.
+  extern "C"
   BOOL WINAPI SetInformationJobObject(HANDLE hJob,
                                       JOBOBJECTINFOCLASS JobObjectInfoClass,
                                       LPVOID lpJobObjectInfo,